Interpretive Summary: Botrytis blossom blight can cause extensive crop loss of rabbiteye blueberry in years when the weather during bloom is cool and rainy. Fungicides are available to control this disease but they are costly and their application is often delayed due to weather. Biological control agents (BCAs) offer an alternative to conventional fungicides for control of Botrytis blossom blight. Previous studies have had success using bees to disperse BCAs. Using cage studies and field trials we showed that bumble bees would carry fluorescent dyes and BCAs to blueberry flowers at the proper time for disease control. When bees distributed the commercial BCA preparation of the fungus, Gliocladium catenulatum, to blueberry flowers the symptoms of blossom blight were less than those on the untreated control plants. Laboratory tests confirmed that the fungus was present on the styles of the flowers that the bees visited. Blueberry researchers, extension agents and growers may use this information as they plan future disease control options.

Technical Abstract:
Botrytis blossom blight caused by Botrytis cinerea may cause severe crop loss in rabbiteye blueberry, necessitating applications of expensive fungicides. Commercial bumble bees, Bombus impatiens, were tested as vectors of the fungicidal biological control agents (BCAs), Prestop® Gliocladium catenulatum and Mycostop® Streptomyces griseoviridis, against blueberry blossom blight. A single bumble bee hive and four flowering blueberry plants were confined within each of ten 1.5-m3 insect exclusion cages. Test products were applied onto the bodies of worker bees exiting each hive through a tubular V-shaped dispenser containing the test product affixed to the hive’s entrance. Dissected floral parts were plated onto agar and recovery frequency of BCAs was determined. Stylar infection rates were 100% for S. griseoviridis and 70% for G. catenulatum. In field trials, bumble bees vectored a UV-fluorescent dye and BCAs to about one third of the open blueberry flower clusters in a small field in as few as eight days. Flowers were inoculated with Botrytis ten days after placing BCA vectoring bee hives in the field. The Botrytis inoculated flowers on bee-visited stems (Prestop treated flowers) had more white corollas (71%) and a lower disease incidence than unvisited flowers (52% white corollas). These results indicate that bumble bees can vector sufficient BCA to blueberry flowers to reduce floral damage caused by B. cinerea.